Senescence is a form of stable proliferative arrest historically associated with the exhaustion of replicative potential of cells. Activated oncogenes, such as ras, can induce senescence prematurely in young cells. Recent studies demonstrate that like apoptosis, oncogene-induced senescence is a bona fide tumor suppressing mechanism in vivo, which needs to be compromised during cancer development. However, the signaling pathways responsible for this important anti-tumorigenic response are poorly understood. Studies from our lab indicate that the p38 MAPK and its downstream substrate kinase PRAK play a key role in oncogenic ras-induced senescence and tumor suppression both in vitro and in vivo, and that PRAK is likely to be a tumor suppressor protein. Our preliminary data reveal that PRAK directly interacts with a multifunctional histone acetyltransferase Tip60. Further investigation indicates that Tip60 is essential for oncogenic ras- induced senescence, and that Tip60 expression and phosphorylation are both induced during senescence induction. Moreover, p38 directly phosphorylates Tip60 on S155 and T158 in cells and in vitro. In addition, increased expression of Tip60 enhances the kinase activity of PRAK, and suppression of Tip60 reduces ras- induced phosphorylation of PRAK at the activation site, suggesting that Tip60-PRAK interaction is important for the activation of PRAK in senescent cells. Together, these findings demonstrate that Tip60 plays a central role in oncogene-induced senescence. We hypothesize that oncogenic Ras activates Tip60 through p38-mediated phosphorylation and protein stabilization, and that activated Tip60 in turn stimulates the kinase activity of PRAK, leading to induction of the tumor-suppressing senescence response. This application is designed to investigate the mechanisms underlying the role of Tip60 in senescence induction and tumor suppression in vitro and in vivo. First, regulation of the phosphorylation, expression and acetyltransferase activity of Tip60 by the Ras-p38 pathway in senescent cells will be examined. Second, the impact of Tip60-PRAK interaction on the expression and activity of each binding partner and p53 during ras-induced senescence will be analyzed. Finally, the contribution of PRAK and Tip60 and their interaction to senescence induction and tumor suppression will be examined in vivo, using mouse cancer models in which tumor development is driven by oncogenic ras, and in human cancers with high frequency of activating mutations in senescence-inducing oncogenes. Results from these studies will delineate the signal transduction pathway mediating the tumor suppressive senescence response, and establish the mechanism underlying the role of Tip60 in oncogene- induced senescence and tumor suppression. The proposed work may lead to the development of new cancer therapies targeting the senescence pathway. PUBLIC HEALTH RELEVANCE: Studies proposed in this application are designed to investigate the mechanism of oncogene-induced senescence, an important tumor suppressive cellular response that is disrupted during human cancer development. These studies focus on a protein kinase PRAK and an acetyltransferase Tip60, both of which are likely to be tumor suppressor proteins. Results from these studies will define the signal transduction pathway mediating the senescence response and tumor suppression, and may lead to the development of novel cancer therapies targeting the senescence pathway.